A well known apparatus for monitoring fluid flow is of the kind comprising means for transmitting acoustic signals in opposite directions through a fluid with components in the direction of flow; means for receiving transmitted signals; and means for sensing the received acoustic signals and for providing a signal representative of the fluid flow velocity.
One example of such an apparatus is illustrated in British Patent Specification No. 1,285,175 in which acoustic signals, usually ultrasonic signals, are transmitted alternately in upstream and downstream directions by a pair of transducers the arrival of a transmitted signal at one transducer causing the other transducer to transmit another signal. This method of transmission is known as "sing-around". The time for a signal to travel through the fluid from one transducer to the other is dependent upon the velocity of the fluid in the direction of transmission, the velocity of sound in the fluid, and the path length. By determining the rate at which each transducer transmits a signal, both the fluid flow velocity and the acoustic sound velocity can be determined. One of the major problems with this apparatus is that no account is taken of the fact that acoustic velocity itself can change due to changes in temperature in the liquid. Furthermore, the read out time for such a flow meter, that is the time required to measure the difference in frequencies to the required accuracy, can be long, especially when low flow rates are being monitored.
A more recent proposal is described in British Patent Specification No. 1,422,791. A pair of variable frequency voltage controlled oscillators are used to cause a series of pulses to be transmitted upstream and downstream, the frequency of each oscillator being varied in accordance with the time of arrival of the received pulse so that a fixed number N of oscillations occurs between the transmission and receipt of a pulse. By comparing the two frequencies arrived at, the fluid flow velocity can be determined. In this case, accurate control of voltage controlled oscillators is very difficult and since the difference in upstream and downstream times of flight is of the order of microseconds, any frequency errors in the voltage controlled oscillators are significant.
Another flowmeter is described in British Patent Specification No. 1,551,744. This specification describes the use of one or more voltage controlled oscillators which are modulated with two signals. One controls the average frequency of the VCO and the other modulates the average frequency up and down corresponding to downstream and upstream measuring directions. An alternative embodiment describes the use of a ramp whose slope is modulated in exactly the same manner using two modulating signals. Although this construction enables the sound speed of the fluid to be compensated for, one of the main difficulties is that the level of the modulation voltage corresponding to the flow velocity information must necessarily be extremely small. Another disadvantage is that the system is unbalanced since a linear time function is converted to an inverse time function. The effect of this is that when measuring flow, by taking the mean of the up and down stream VCO frequencies or ramp rate, this mean is not the same as when a zero flow condition exists.